HttpSerializers client set Accept header

[Scottmitch]

Motivation:
HttpSerializers currently set the Content-Type header
but not the Accept header. The Accept header provides
the server a hint as to what type of contnet should
be returned.

Modifications:

• Modify implementations in HttpSerializers to set the
  Accept header

Result:
Clients provide info to servers about what type of
content is accepted in a response.

[Scottmitch]

a trade-off here is if folks want to accept different/multiple payload types (e.g. to handle errors differently) we will overwrite the header value here. we have some options:

• use set(folks could manually adjust headers if necessary, however serializers don't compose as nicely, but maybe safer to be explicit about accept values)
• use add (if there is preexisting type that the user intends to override then we may keep ACCEPT headers that aren't really supported)

folks could also create their own serializers if they don't want this accept value forced on them...

[idelpivnitskiy]

While HTTP allows multiple header entries with the same name, there is a risk that some other peers won't recognize it the same way as a single Accept header with a list of values.

Users can also insert a filter, that will set the correct values after serialization. Not good to split the logic to override our value though. We can also consider adding an overload for all special serializers (like formUrlEncodedSerializer) that takes a user-defined function for headers. WDYT?

[tkountis]

I would expect this to be a user-choice vs an library one.
@idelpivnitskiy's suggestion makes sense, I believe we can make an effort to "add" a header (I am leaning towards add), but any user configuration should take precedence. Creating your own serializer is still an option, but I think its too much to ask for just setting ACCEPT headers.

[Scottmitch]

I think it is safer to start with set for now bcz that is all we know for sure is supported here. We have options to evolve as described above (filter - not ideal to disconnect code, allow users to customize serializer add/set behavior).