NIFI-5258 - Changed addHeader to setHeader which stops X-Frame-Option…

[thenatog]

…s being added twice to responses. Added unit test.

Thank you for submitting a contribution to Apache NiFi.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• Does your PR title start with NIFI-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• Has your PR been rebased against the latest commit within the target branch (typically master)?

• Is your initial contribution a single, squashed commit?

For code changes:

• Have you ensured that the full suite of tests is executed via mvn -Pcontrib-check clean install at the root nifi folder?
• Have you written or updated unit tests to verify your changes?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• [] If applicable, have you updated the LICENSE file, including the main LICENSE file under nifi-assembly?
• If applicable, have you updated the NOTICE file, including the main NOTICE file found under nifi-assembly?
• If adding new Properties, have you added .displayName in addition to .name (programmatic access) for each of the new properties?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.

[alopresto]

Reviewing...

[alopresto]

I verified this in both default and secured modes against a URL that previously returned the double header value:

hw12203:/Users/alopresto/Workspace/nifi (pr2791) alopresto
🔓 0s @ 15:36:56 $ curl -v http://localhost:8080/nifi/
*   Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 8080 (#0)
> GET /nifi/ HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.43.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Date: Wed, 13 Jun 2018 22:37:12 GMT
< X-Frame-Options: SAMEORIGIN
< Content-Type: text/html;charset=utf-8
< Vary: Accept-Encoding, User-Agent
< Transfer-Encoding: chunked
< Server: Jetty(9.4.3.v20170317)
<

hw12203:...space/nifi/nifi-assembly/target/nifi-1.7.0-SNAPSHOT-bin/nifi-1.7.0-SNAPSHOT (pr2791) alopresto
🔓 0s @ 16:10:05 $ curl -v -k --cert conf/CN\=cert-user_OU\=Apache_NiFi.p12:passwordpassword --cert-type P12 https://localhost:9443/nifi/
*   Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 9443 (#0)
* Client certificate: cert-user
* TLS 1.2 connection using TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384
* Server certificate: nifi.nifi.apache.org
* Server certificate: nifi-ca.local
> GET /nifi/ HTTP/1.1
> Host: localhost:9443
> User-Agent: curl/7.43.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Date: Wed, 13 Jun 2018 23:11:26 GMT
< X-Frame-Options: SAMEORIGIN
< Content-Type: text/html;charset=utf-8
< Vary: Accept-Encoding, User-Agent
< Transfer-Encoding: chunked
< Server: Jetty(9.4.3.v20170317)
< 

hw12203:...space/nifi/nifi-assembly/target/nifi-1.7.0-SNAPSHOT-bin/nifi-1.7.0-SNAPSHOT (pr2791) alopresto
🔓 2s @ 16:11:28 $ curl -v -k --cert conf/CN\=cert-user_OU\=Apache_NiFi.p12:passwordpassword --cert-type P12 https://localhost:9443
* Rebuilt URL to: https://localhost:9443/
*   Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 9443 (#0)
* Client certificate: cert-user
* TLS 1.2 connection using TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384
* Server certificate: nifi.nifi.apache.org
* Server certificate: nifi-ca.local
> GET / HTTP/1.1
> Host: localhost:9443
> User-Agent: curl/7.43.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Date: Wed, 13 Jun 2018 23:12:37 GMT
< X-Frame-Options: SAMEORIGIN
< Content-Type: text/html;charset=utf-8
< Vary: Accept-Encoding, User-Agent
< Content-Length: 1058
< Server: Jetty(9.4.3.v20170317)
<

Ran contrib-check and all tests pass. +1, merging.