Fix test/slow/ext_gd/imageweb.php

Summary: @​rrh made me aware that the test was not passing any more with a more recent version of libvpx.
This commit corrects the test and should be robust enough for any future update.

Instead of just printing the header and the file size, it now checks if the header is valid and the specified length matches the real length of the file.

Fixes #3927
Closes https://github.com/facebook/hhvm/pull/3929

Reviewed By: @JoelMarcey

Differential Revision: D1603569
Fix NumberFormatter::format for string values

Summary: The parameter type would be checked instead of the type after converting it to a number (toNumeric)
That would result in having a type of TYPE_DEFAULT instead of the derived type from the parameter hence always trying to format the Int64 value on strings

Also make NumberFormatter::create's third parameter optional.

Fixes  #3936
Closes https://github.com/facebook/hhvm/pull/3938

Reviewed By: @JoelMarcey

Differential Revision: D1605804