added checks for GRPC because it expects ASCIIString

[dhilpipre]

Overview

When the Java Agent attempts to read headers from io.grpc.netty.GrpcHttp2HeadersUtils.GrpcHttp2RequestHeaders it throws an exception because the get method expects an io.netty.util.AsciiString input but receives a String from the agent.

Related Github Issue

This fixes
#2177

This code check to see if the Http2Headers class that is passed into the constructor for com.agent.instrumentation.netty4116.Http2RequestHeaderWrapper is GrpcHttp2RequestHeaders. If it is it sets a flag.
In both the getHeader and getHeaders methods, it checks if the GRPC flag is set and if true then creates an AsciiString to use.

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