tp</span><span class="special">::</span><span class="identifier">header</span></code></a> and <code class="computeroutput"><span class="identifier">value_type</span></code>
          objects. This enables the composition of body types. The previous single-argument
          constructors are deprecated and will be removed in the next version. Actions
          required: Change user-defined instances of <a class="link" href="concepts/BodyReader.html" title="BodyReader"><span class="emphasis"><em>BodyReader</e