We've had some annoyances with properly looking up headers from an HTTP

response in hosting service implementations in a consistent manner

across versions of Review Board, and across versions of Python. Work has

been done to fix this for GitHub, where this was more immediately

noticed.
This change works to standardize how we store headers, both for requests

and responses, in the new hosting service HTTP implementation. We now

store all headers as lowercase, and normalize on lookup.
HostingServiceHTTPResponse now has a get_header() method, just like

HostingServiceHTTPRequest. This is now preferred over accessing

headers directly, and will handle normalization on the caller's

behalf. This is future-proof and avoids any further confusion around

headers.

Unit tests pass on Python 2.7 and 3.6 through 3.8.