std::string attribute(const std::string name_ ) signature

[wizard29]

Hi.

The std::string Magick::Image::attribute(const std::string name_ ) looks like a mistake.
I think std::string Magick::Image::attribute(const std::string& name_ ) const will be correct if no internal data is changed inside the function.

[dlemstra]

Thank you for reporting this, I have also added const to the following methods:

- bool antiAlias(void)
- size_t colorMapSize(void)
- size_t totalColors(void)
- std::string artifact(const std::string &name_)
- ImageMoments moments(void)
- Image separate(const ChannelType channel_)
- Image uniqueColors(void)

Please let me know if you think I missed another one.