Faster and simpler replace in SSB

The original method was adding a suspicious space that was eventually
removed with a series of 3 calls to change.

This should not be necessary. I have tested this with various gap
positions and lengths, for all replace cases I could think of.

The test can not be added to the CTS as it would need to expose the
internal resizeFor and moveGapTo methods.

Change-Id: I194457fbcfd758fa69a7f380665cfd5ae4d3f1d4