Spring Fix common HashMap initial capacity issue in HttpMethod

It is a common error to specify HashMap's initial capacity as expected size. It doesn't take load factor into consideration, which is never 1.0. Fixed the issue in HttpMethod via the util method from guava library.

Comment From: chenqimiao

@NathanQingyangXu The final mapping.size is 8, there seems to be no problem setting the initialization to 16.

Comment From: NathanQingyangXu

@NathanQingyangXu The final mapping.size is 8, there seems to be no problem setting the initialization to 16.

Yeah, saw that. thanks for your feedback. I closed this PR.