add len parameter in update() of resty.md5

[sjnam]

When I use _M.update(self, s) in resty.md5, I had an case when len is not #s.
so, how about the following?

function _M.update(self, s, len)
return C.MD5_Update(self._ctx, s, len or #s) == 1
end

[agentzh]

@sjnam Thanks for the patch! But will you add a test case to the existing test suite too? Thanks!

[sjnam]

@agentzh I added a test case. Thanks!

[agentzh]

@sjnam Awesome! I'll look into this as soon as I can manage :) Thanks a lot!

[agentzh]

@sjnam Mind to add a note to README as well? ;)

[sjnam]

@agentzh
I just add one comment line and small change to already given test code in README.

[agentzh]

@sjnam Cool. Thanks!