Fixes call to equals() comparing different types

Motivation:
Sonar points out an equals comparison, where the types compared are different and don't share any common parent
http://clinker.netty.io/sonar/drilldown/issues/io.netty:netty-parent:master?severity=CRITICAL#

Modifications:
Converted AsciiString into a String by calling toString() method before comparing with equals(). Also added a unit-test to show that it works.

Result:
Major violation is gone. Code is correct.