refactor conditional header field descriptions

[julian.reschke@…]

The current definitions for If-Match and If-None-Match could be simplified by stating exactly once what the expressed condition is, and then just refer to the outcome of this text in further discussions.

Such as

"The If-Match condition is met if and only if any of the entity-tags listed in the field value match (as per Section 2.3.2) the entity-tag of the selected representation for the target resource, or if "*" is given and any current representation exists for the target resource."

Then the following text could be simplified from:

"If none of the entity tags match, or if "*" is given and no current entity exists, the server MUST NOT perform the requested method, and MUST return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating method, such as PUT, from modifying a resource that has changed since the client last retrieved it."

to

"If the condition is met, the server MUST NOT perform the requested method, and MUST return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating method, such as PUT, from modifying a resource that has changed since the client last retrieved it."

(The effect for If-None-Match is bigger because the description is slightly more complex)

[julian.reschke@…]

Proposed patch

[julian.reschke@…]

Proposed patch

[julian.reschke@…]

From [1761]:

Refactor If-*Match header fields to separately define the condition (see #372)