TS-2531: add a nohost_rule check for proxy request

This change is used to solve the problem TS-2531. 

Below is my test method:

1. config remap.config as below:
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map / http://127.0.0.1:8885/
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(127.0.0.1:8885 is my backend server.)

2. start this new ATS:
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sudo  ./bin/traffic_server --httpport 80:fd=7,443:fd=8:ssl
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3. use command "telnet localhost 80" to send a request like TS-2531:
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GET http://localhost/test
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Now , a 200 OK response returns.