Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

When the uwr_get_http_content_str() function, which returns the request
body content as a string, was added it used uwr_get_http_content_len()
to determine the length of the returned string (the request body content
is not null-terminated).

This could potentially in some circumstances return too much data if the
request was split over multiple calls into the Wasm module and
uwr_get_http_content_str() was called before all the data had actually
been received.

Instead use the newly introduced uwr_get_http_total_content_sent()
function to determine the amount of data to return in the string as it
currently stands.

Fixes: bf968c9 ("Rust/rusty: Add uwr_get_http_content_str()")
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

This returns the total amount of content that the Wasm module has
received so far.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

This may not matter in rust (if you use a function before the compiler
has seen a definition for it), but anyway in preparation for adding a
uwr_get_http_total_content_sent() function, put the content length
related functions before the functions that return the content.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

This adds the following convenience functions for adding HTTP response
headers, Content-Type & Content-length

  uwr_http_add_header_content_type(ctx: *mut luw_ctx_t, ctype: &str);
  uwr_http_add_header_content_len(ctx: *mut luw_ctx_t);

These are perhaps the two most common headers so it makes sense to
reduce the effort to adding them.

E.g before

  uwr_http_add_header(&ctx, "Content-Type", "text/plain");
  uwr_http_add_header(
      ctx,
      "Content-Length",
      &format!("{}", uwr_get_response_data_size(ctx)),
  );

after

  uwr_http_add_header_content_type(ctx, "text/plain");
  uwr_http_add_header_content_len(ctx);

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

This function is like uwr_get_http_content() except that it returns a
Rust str.

This is more convenient if you want to operate on the body content
within Rust.

It's worth noting that uwr_get_http_content() returns a non null
terminated buffer which makes it tricky to work with in Rust.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

luw_http_add_header() no longer takes an idx argument.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

The various uwr_get_ functions should take the context pointer as a
const.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>