Update Request.php to add LINK/UNLINK HTTP methods

[amreladawy]

LINK and UNLINK need to be added to allowed methods.

[coveralls]

Coverage remained the same at 97.954% when pulling 448234d on amreladawy:patch-1 into c138554 on slimphp:3.x.

[johnhunt]

Please put LINK in alphabetical order in the array to maintain consistency.

[akrabat]

Sorry, but a far better solution is to remove the list of validMethods altogether and change filterMethod() to check for a regex of the form:

/^[[:alnum:]!#$%&\'*\/+\-.^_`|~]+$/i

and only throw the InvalidMethodException if the $method doesn't match that regex.