Header optimizations and header key normalization optional.

(In relationship to https://github.com/4thline/cling/pull/61 )

Optimized toString() method to use a single StringBuilder object.

readLine() can now receive a StringBuilder object so that it can reuse
that instance across recursive calls, this had an important impact
on DalvikVM.

New Header(boolean normalize) constructor to make header key normalization
optional. This change is related to changes on cling
See https://github.com/gubatron/cling/commit/bd9991b06c09b8f82070f3da639980676dd9fb2f

updates on the normalize method to not make repeated calls to .length()
which has a performance hit on DalvikVM.