I'm a native English speaker and EFL teacher, this really made me question myself!
If I had rushed to say something, I would have said 'an' before any word starting with a vowel. Though it is actually 'an' before anything that starts with a vowel 'sound'.
History definitely does not "officially" start with a vowel sound in any form of spoken English that is taught, so, 'officially' 'history' really can't be preceded by 'an'.
I think 'an' is sort of special as a word, because more so than other words, its form of use in grammatical written English, is influenced by the practicalities of spoken English. Really, as anybody would know who spends any sort of time studying a language; reading, writing, listening, and speaking are very different experiences.
My teaching Engish teacher [ not a typo ], went as far to say that really when you learn 'a language', you are learning two languages; the 'written language' and the 'spoken language'. With the spoken language, you would not pronounce each word in a phrase as you would pronounce it in isolation, the sound of a word in a phrase is dictated by the words that come before and after it, and what precisely it is you want to communicate. So when we speak a sentence, it is one long continuous sound or entity, or an 'utterance'. When things are written, the component words are obviously written with spaces inbetween them and exist graphically as clearly defined separate entities. This may be all very obvious (at least after being outlined like this), but it's important to consider, when considering the use of the word 'an'.
Often when speaking people take short cuts verbally to make things easier to say e.g 'got to' becomes 'gotta', 'is not' becomes 'isn't' becomes 'ain't', it is not official language, or commonly taught language, but this of course doesn't make it less real or have less meaning. Sometimes phonemes/sounds/letters are dropped altogether as the meaning that the sound communicates can be derived from the rest of the phrase or the context of the situation.
I myself, due to my families being from different backgrounds use two accents, or forms of spoken English: Received Pronunciation / Estury English / English English . . . and . . . Cockney / Working Class London spoken English (or accent, call it what you will). My mother's side of the family speak RP, and some of my relatives on her side sound more aristocratic than the Royal Family. My father's side are of a working class London background and talk like the Orcs from 'Lord of the Rings'. It is almost comical when we have a family get together.
Sooooo, why do I say all this?
When I talk in my Cockney accent, I do not pronounce some vowels at the end of words: 'cat' becomes 'ca', or a lot of the time 'h' at the beginning of words. . . .
. . . . So in Cockney speech, I would say 'an 'istorian'. 'An', because it is a lot easier to connect or combine the words into an utterance in spoken English, when you have a consonant ('n') between the two vowel sounds.
. . . but, 'istorian' is not how you are taught to pronounce the word 'historian' in the commonly taught Received Pronounciation version of British English speech. Whether this is also defined as 'official' English speech, I do not know.
I imagine 'istorian' is not how the pronunciation is taught in other English speaking nations either.
Official/common language evolves a lot of the time through chance, mutation and mistakes. Sometimes how people commonly percieve the correct pronunciation of the written form of a language, influences how we commonly say something. Sometimes written language is influenced by how people commonly say something.
So all in all, even though it makes part of my spoken language cultural heritage less represented, I have to say there seems to be more support for 'a historian' to be officially grammatically correct, than 'an historian', as I can not think of any circumstance where it would be thought of as a good idea for the initial teaching of the pronunciation of the word 'historian', to be taught as 'istorian'.
'Historian' overwhelmingly does not start with a vowel sound, where you would have someone pronouncing the word in isolation, in a clear a way as possible in order for its meaning to be understood.
[Shoot, was not meant to spend all this time writing this!!!]