Further optimize declaration parsing

[glassfishrobot]

After #81 there is still some optimization potential left in XML declaration parsing. XMLDeclarationParser seems to require a PushbackReader with a buffer size of 4096.This seems excessive to unread a declaration which is 35 characters in the common case. However reducing the buffer size causes tests to fail. Upon closer inspection the parsing algorithm seem to be:

1. read the entire first node
2. check if the node starts with <?xml, if not unread the entire node

This has two issues. The first issue is a functional one. The algorithm works only if the buffer is large enough to unread the entire node. The tests contain some XML files where the first node is a large comment node. This seem to be where the number 4096 comes from. However it could easily that the first node is larger and then parsing will fails with an exception. The second issue is related to performance. After reading 12 characters we can already decide if the node is an XML declaration. If not we can stop right there. This reduces the required size of the buffer in the PushbackReader to 12. More than two orders of magnitude smaller. Based on this the algorithm we propose is:

1. read up to 12 characters or the whole node which ever happens first
2. check if the node starts with <?xml, if not unread what you have read

The number 12 comes from the fact that <?xml in UTF-16 with a BOM is 12 bytes.

[glassfishrobot]

Reported by braghest

[glassfishrobot]

Was assigned to gagordon

[glassfishrobot]

This issue was imported from java.net JIRA SAAJ-83