Evaluate dynamic fields in event when used with gsub

[suyograo]

Closes #1529

[elasticsearch-release]

🔴 Test failed.
Refer to this link for build results: http://build-eu-1.elasticsearch.org/job/Logstash_PR/34/

[jordansissel]

LOOKS GOOD TO US.

[suyograo]

retest this please

[elasticsearch-release]

💚 Test passed.
Refer to this link for build results: http://build-eu-1.elasticsearch.org/job/Logstash_PR/44/

[elasticsearch-release]

🔴 Test failed.
Refer to this link for build results: http://build-eu-1.elasticsearch.org/job/Logstash_PR/45/

[elasticsearch-release]

💚 Test passed.
Refer to this link for build results: http://build-eu-1.elasticsearch.org/job/Logstash_PR/46/