URL-encode the :path header.

SPDY spec says the :path header value must conform to a valid URL, which
requires the path, query, and fragment to be percent-escaped. NSURL
returns the original, non-escaped, version of the path, but not the
query or fragment.
Unit tests for NSURLRequest+SPDYURLRequest.m methods.
Merge branch 'path_urlencoding_fix' of git://github.com/kgoodier/CocoaSPDY into kgoodier-path_urlencoding_fix

Conflicts:
	SPDY.xcodeproj/project.pbxproj
Merge branch 'kgoodier-path_urlencoding_fix' into develop