Struct iron::headers::IfUnmodifiedSince  
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        [src]
pub struct IfUnmodifiedSince(pub HttpDate);
If-Unmodified-Since header, defined in
RFC7232
The If-Unmodified-Since header field makes the request method
conditional on the selected representation's last modification date
being earlier than or equal to the date provided in the field-value.
This field accomplishes the same purpose as If-Match for cases where
the user agent does not have an entity-tag for the representation.
ABNF
If-Unmodified-Since = HTTP-date
Example values
Sat, 29 Oct 1994 19:43:31 GMT