StringValues.Count test null first

[benaadams]

Trying to cast is string and then doing a is null later is inefficent when in practice a lot of the values are default(StringValues); so it should do the cheap null test first before the cast

[ghost]

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Issue Details

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Trying to cast is string and then doing a is null later is inefficent when in practice a lot of the values are default(StringValues); so it should do the cheap null test first before the cast

Author:	benaadams
Assignees:	-
Labels:	area-Extensions-Primitives
Milestone:	-

[eerhardt]

LGTM, as long as:

when in practice a lot of the values are default(StringValues)

is true more times than StringValues represents a single string.

[benaadams]

is true more times than StringValues represents a single string.

If header is not set IHeaderDictionary returns default and out of the range of possible headers they are usualyl not set