Client: lua

The http transport client breaks when the traffic is routed through a proxy that changes the casing of the header keys. This is not only incorrect but also different from the implementation in all other languages.
This pull request changes this, so http headers are now stored/retrieved in a case insensitive matter.
Since there are no unit tests for lua (that i could find?) I did not add tests that isolate this bug, but it's quite obvious from looking at the source.

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