HTTP plugin match request header with out using ^ and $

[idanto]

Check CONTRIBUTING guideline first and here is the list to help us investigate the problem.

Describe the bug
In HTTP input plugin there is a switch/case part

fluentd/lib/fluent/plugin/in_http.rb

Line 334 in 5ef1c03

when /Expect/i

This switch case is based on regular expressions that are not an exact match - not start with ^ and ends with $.
In case I have a custom header with one of those values like x-envoy-expected-rq-timeout-ms (which is auto-generated by envoy proxy) the request will fail although it should pass.

To Reproduce
Setup Fluentd with HTTP input plugin and send a custom header called some-header-expected-something with a value which is not 100-continue.

Expected behavior
The match should be an exact match.

Your Environment

• Fluentd or td-agent version: fluentd 1.6.3 (but also true to the latest version)
• Operating system: alpine 3.9.4
• Kernel version: 4.14.138-114.102.amzn2.x86_64

Your Configuration

<system>
  log_level "#{ENV["LOG_LEVEL"] || 'info'}"
</system>

<source>
  @type  http
  @id    input1
  port   "#{ENV["PORT"] || '24224'}"
  bind 0.0.0.0
</source>

<match "**">
  @type stdout
</match>

Your Error Log
No error log just 417 respose code

HTTP/1.1 417 Expectation Failed
content-length: 0
content-type: text/plain
x-envoy-upstream-service-time: 4
date: Sun, 29 Dec 2019 14:27:17 GMT
server: istio-envoy