Fused constructors of `MethodNotAllowed` into one.
Fused constructors of `MethodNotAllowed` into one.

There is no good reason to provide two c-tors for `MethodNotAllowed`,
with one taking `requestMethod` and one not. Instead, an `Option<>`
can be used. This also removes the need for copy-paste in the c-tor
body.

Review: https://reviews.apache.org/r/68945

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