Remove TBLDECWWWCOMP_ constant before redefining it.

When using tire under ruby 1.8 the following warning is logged (twice as shown).
```bash
...uri_escape.rb:14: warning: already initialized constant TBLENCWWWCOMP_
...uri_escape.rb:15: warning: already initialized constant TBLENCWWWCOMP_
```

This pull request removes the constant before adding to quiet the warning.
It and removes the duplicate init.