Support key as integer for the maps put.

[zhuangsirui]

Support key as integer for the maps put. Add test case at bson_tests:maps_put_test/0.

[comtihon]

I don't like it.
The idea is, that you get what you put, so if you put #{1024 => <<"integer">>} - you shoud get #{1042 := <<"integer">>} from bson. Not #{<<"1024">> => <<"integer">>}.
For quick I can advice you to turn integer keys to binaries outside bson, because it is not bson-related question.
Also, if you are using just bson and want this function - you should add ability to receive integer keys as well. But - if you are using bson with mongodb - this will not work, as mongo do not like integer keys.

[zhuangsirui]

I don't think this will be a problem.
Because now when I put #{atom => <<"atom">>} - I get #{<<"atom">> => <<"atom">>} as same as the integer.
The BSON key only can be a string when I decode from a BSON, this is the BSON problem. But I think when I encoding something in a specified language, the library should take care of this stuff.
I really want you to take this PR, but it's fine if you still don't like it. I'm just do this thing outside BSON :(

[comtihon]

atom was left for old driver versions, which use atom keys, not to break them. I plan to remove it in future.