Fixed SearchKeys method returning keys with KeyPrefix included

[carlosboeing]

I ran some tests to make sure the string manipulation on the keys list won't add too much overhead to the method.

Here are the results:

100K records:
with prefix 326ms
without prefix 305ms

500K records:
with prefix 1620ms
without prefix 1589ms

1M records:
with prefix 3176ms
without prefix 3139ms

[imperugo]

Thanks.
I'm going to review, merge and publish it tomorrow.

[imperugo]

On nuget now.

Thanks again