Fix header name mismatching after rev.14870

Fix header name mismatching after rev.14870

When a mime header set contains two custom headers and one
name is the prefix for the other the name lookup using a
fixed length for String comparison can wrongly match the
longer header as being equal to the shorter one, since only
the identical prefix portion is compared.

To avoid this we must check that the lengths are also matching.

This also improves performance very slightly as the common
case for custom headers is to have an "X-" prefix which is
slower to compare than total length. Headers having same
length and same prefix is quite rare.