net/http: customize ServeMux with not found and not allowed handlers

[hossein1376]

Before, NotFound and MethodNotAllowed handlers were hard coded.

With this change, ServeMux behaviour can be customized, and by using optional function parameters, backward compatibility is ensured as well.
Two new functions, namely WithNotFoundHandler and WithNotAllowedHandler, are introduced which can be used as arguments to the NewServeMux func.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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[gopherbot]

Message from Damien Neil:

Patch Set 1: Hold+1

(1 comment)

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