Rename [method] to _method.

This makes the more commonly used `_method` option congruent with other
'special keys' like _ssl and _host.

Include some backwards compatibility for creating routes, and generating
URLs.
Remove the header matching from Route::parse()

This feature was very infrequently used to the point that many core team
members did not even know it existed. Removing it makes sense now that
custom route classes are well supported and easy to use.
Fix incorrect doc block.
Merge pull request #3987 from cakephp/3.0-method-rename

Rename [method] to _method.