com/sun/net/httpserver/Headers.java: Ensure mutators normalize keys and disallow null for keys and values

The Headers class normalizes its keys to adhere to the following format: First character uppercase, all other characters lowercase, for example "foo" -> "Foo". This behaviour is not consistent across the mutator methods of the class, in particular putAll() and replaceAll() do not apply normalization.

Suggested fix:
- change the implementation of putAll()
- add adequate override of the java.util.Map default method replaceAll()

Additionally, update Headers to disallow null values for keys and values.