Body constructor doc examples

[jaemk]

Addresses libz blitz (#120) "Examples for the Body constructors", issue #116

[seanmonstar]

Thanks for the pull request! Just one thought about the text of the sized constructor.

[seanmonstar]

Thanks for the pull request! Just one thought about the text of the sized constructor.

[seanmonstar]

I think the example probably should address the point in the lines above, about how this constructor is useful if "we don't want to load the data in memory". If someone has a a bundle of bytes, it's actually better in to use Body::from().

This was originally added to allow someone to include the length with a File or similar.

let file = File::open("foo.txt")?;
let len = file.metadata()?.len();
let body = Body::sized(file, len);

[jaemk]

Woops, that makes more sense!

[jaemk]

• The sized constructor example is updated to use a File and it's Metadata.len()
• The new constructor docs are updated to point out the available From impls that should be used for a set of known bytes.

[seanmonstar]

Thanks!