perf(Request): Cache native stream check

Avoid having to call isinstance every time a new Request object
is initialized. Use a flag that can be set after the first time
this check is made, so that thereafter a simple boolean operation
is required instead of having to call the more costly isinstance
function.
Merge pull request #363 from kgriffs/cache_wsgiref_check

perf(Request): Cache native stream check
chore(AUTHORS): Added ealogar. Thanks for contributing!