Improve django.http types

[noelleleigh]

This PR adds polish to several classes in the django.http module:

• django.http.request.HttpHeaders
• django.http.request.HttpRequest
• django.http.response.ResponseHeaders
• django.http.response.HttpResponse
• django.http.response.StreamingHttpResponse
• django.http.response.FileResponse
• django.utils.datastructures.CaseInsensitiveMapping

The changes broadly fall under:

• More specific typing (replace Any with specific types, remove = ... for class attributes that are always defined, use typing.Never for methods and properties that always raise an exception)
• Add missing methods and properties
• Move methods and properties to the correct classes

Some of the changes here could definitely be interpreted as stylistic rather than functional, and I'll happily make any changes that are requested.

[sbdchd]

Thank you!!