Use Better API for Case Insensitive String Equality Check

[munahaf]

In file: header_propagate.go, there is a function findField that performs a case insensitive String equality check. String comparison with strings.ToLower or strings.ToUpper is significantly more expensive. For example, strings.ToLower will have to allocate memory for the new strings, and then convert the string to lower case. It is better to do the case insensitive equality check using the strings.EqualFold(https://pkg.go.dev/strings#EqualFold) function. This function does not need to create two intermediate strings and can return as soon as the first non-matching character has been found. I fixed the issue by introducing the strings.EqualFold function.

Sponsorship and Support:

This work is done by the security researchers from OpenRefactory and is supported by the Open Source Security Foundation (OpenSSF)(https://openssf.org/): Project Alpha-Omega(https://alpha-omega.dev/). Alpha-Omega is a project partnering with open source software project maintainers to systematically find new, as-yet-undiscovered vulnerabilities in open source code - and get them fixed – to improve global software supply chain security.

The bug is found by running the Intelligent Code Repair (iCR) tool by OpenRefactory and then manually triaging the results.