Added note on behavior of custom headers in HAR files

[dledfordcf]

Custom headers wont show up in a HAR file, this can cause confusion since it appears the header isnt being injected. Added clarification on this and how to test if the header is being injected.

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[maxvp]

Hi @dledfordcf, thanks for the contribution. I've applied some PCX style guide edits and approved this.

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