Add always seekable body plugin

[joelwurtz]

Q	A
Bug fix?	yes
New feature?	yes
BC breaks?	no
Deprecations?	no
Related tickets	fixes #98, replace #99, partially #97
Documentation	php-http/documentation#251
License	MIT

What's in this PR?

Add a plugin that ensure request body or response body is always seekable

Why?

This may be needed when multiple librairies need to acces the content but are not aware of each other, so this allow them to read the body once and then rewind it.

[dbu]

very cool! i have a comment on the testing, and a minor spelling nitpick on the phpdoc

[Nyholm]

Good.

Do I want to decide if this should make only the response seekable?

[dbu]

hm, good question. it could make sense to distinguish that, because i assume adding the buffer is a significant cost that should be avoided when its unnecessary. rather than having flags, i would prefer to split the plugins into SeekableRequestBodyPlugin and SeekableResponseBodyPlugin

(either way i think we can drop the "Always" from the name, it is more verbose without adding much more clarity imho)

[joelwurtz]

As you wish, we can also spli this into 2 plugins : RésponseBodySeekable and RequestBodySeekable, which may be better since you will not put this 2 plugins at the same place :

• RequestBodySeekable should be in first so following plugins can seek its body
• ResponseBodySeekable should be in last so previous plugins will correctly have a seekable body on the response

[dbu]

didn't even think of that, but yes, lets split the plugin then.

[joelwurtz]

Should be good, will update the documentation accordingly if this is good to merge

[dbu]

looks good to me!