Fix answer_type::get_header().

The previous implementation returned a reference to an out-of-scope string.

include/answer.hpp

@@ -103,7 +103,9 @@ struct answer_type
     /*!
      *  @brief  Returns the value of a header field.
      *
-     *  Case insensitive, only ASCII.
+     *  Is only valid for as long as the answer_type is in scope.
+     *
+     *  @param  field Case insensitive, only ASCII.
      *
      *  @since  0.1.0
      */

src/answer.cpp

@@ -54,7 +54,7 @@ string_view answer_type::get_header(const string_view field) const
         auto pos{static_cast<size_t>(it - headers.begin())};
         pos = headers.find(':', pos) + 2;
         const auto endpos{headers.find('\n', pos)};
-        return headers.substr(pos, endpos - pos);
+        return string_view(&headers[pos], endpos - pos);
     }
 
     return {};