Fix repeated headers to be bounded properly

[Empact]

Previously, the field lookup occurred from the beginning of the line so would repeated find the same line location if the same string or a substring of a header was searched more than once.

Instead track the current end index of the previous field to base the search state location on.

[neekey]

Looks good! Thanks

[Empact]

@neekey once you make a release, I'll update neekey/ps#69 to reference it