ensure mode is always set to INITIAL during startTokenization (#119)

While currently, all branches set a mode, this is both subtle and fragile; if a
branch ends up not setting mode it will never be reset. This means
subsequent calls to start tokenisation can end up with a different
initial mode.

This change simply resets mode to INITIAL on every startTokenization() call,
ensuring that if we ever miss it, it will be reset.

Author: keithamus

src/nu/validator/htmlparser/impl/TreeBuilder.java

@@ -592,6 +592,7 @@ public boolean dropBufferIfLongerThan(int length) {
         templateModeStack = new int[64];
         listOfActiveFormattingElements = new StackNode[64];
         needToDropLF = false;
+        mode = INITIAL;
         originalMode = INITIAL;
         templateModePtr = -1;
         stackNodesIdx = 0;