Performance: reserve body to avoid frequent reallocations and copies

[ArnaudBienner]

The append I'm trying to avoid is the one at line 7507:
res.body.append(buf, n);

[ArnaudBienner]

Sorry for the mess with the other PR.

Hopefully this will make more sense now :) This patch, but also the alternative approach I made in #1782.

[yhirose]

@ArnaudBienner same as #1781 (comment), I don't think it improve performance since std::string::append won't be called after your code.

[ArnaudBienner]

@ArnaudBienner same as #1781 (comment), I don't think it improve performance since std::string::append won't be called after your code.

Isn't it called as part of out which is passed to detail::read_content along with res and used to read the data?

[yhirose]

@ArnaudBienner sorry for the late reply. You are right. res.body.append gets called after your code. But the code shouldn't be called when a content receiver is provided, because res.body should be empty in this case.

    auto res = cli.Get("/hi", [&](const char *data, size_t data_length) {
      ...
      return true;
    });