HttpRequest is a pretty old class, and was created before we had a lot

of our current code and documentation standards in place. As it evolved

to be compatible with Python 2.x and 3.x, we ended up with some (mostly

harmless) inconsistency in its behavior on these versions that weren't

caught by the very minimal test suite we had in place
This change fleshes out HttpRequest, adding docs, improving the

readability of some code, and bringing consistency in how it handles

string types. String types for form fields and files are now normalized

to byte strings when setting them, rather than accepting any string type

and leaving it up to the encoding stage to deal with them. This gives us

something stable we can work with.
Small discrepencies like the ordering of files and fields have been

resolved. They're now encoded in insert order, rather than Python

dictionary iteration order (which varies across 2.x and various 3.x

releases). Our old unit tests didn't catch this.
This also trims the fat, getting rid of field/file deletion methods,

which were never used anywhere and, in the case of file deletion, had

a misleading argument.
The unit test suite has been fleshed out some, doing a better job of

testing normalization and building of various fields and testing the

actual encoded payload instead of relying on a hand-crafted and

simplistic form-data parser.

Unit tests passed on Python 2.x and 3.x.
Used these changes to post this review request.