Filtering fails when target contains multiple escapes

If I try to interpolate `##${first} and #${last}` where `#`  is the escape string, the output is `#${first} and #{last}` and not `#${first} and ${last}`. See commit 8ccb8a953519950019e1a97322960c8fddcf9530 for a test case.

This issue comes from https://issues.apache.org/jira/browse/MASSEMBLY-756.
