OPENDJSDK-85 AVA does not provide a way to get back the user provided attribute type

The solution consists in adding AVA.getAttributeName().

AVA.java:
Added attributeNakme field + getter + modified toString()
Addeed constructor AVA(AttributeType, String attributeName, Object attributeValue).
In readAttributeName(), changed return type from AttributeType to String.
Improved javadoc, by ading a note about the name of this class.
Extracted method illegalCharacter().
Simplified needEscaping().

AVATestCase.java: ADDED

ByteString.java:
Made toHexString() compatible with valueOfHex() - necessary to have a correct AVA.toString()

ByteStringTestCase.java:
Added a test for encoding/decoding as hex.

CollationSubstringMatchingRuleTest.java:
Consequence of the change to ByteString.toHexString().