HeadersFrame: Add a method to check for end of stream
HeadersFrame: Add a method to check for end of stream

Instead of clients having to check the state of the flag, they can
simply use this convenience method to check whether the headers
frame represents the end of a stream.

mlalic/solicit,jinnjuice/solicit,gwicke/solicit,tempbottle/solicit,stepancheg/solicit,Galvanization/solicit