Fix broken std::string assign()

[lglina]

The data member variable in vector was previously renamed to _data so we could have a data getter function (see f379188 ). Not all references in string were updated, so in some cases we're trying to pass the data function to char_traits copy/move/assign where it expects a char_type* instead, breaking things like std::string assign().

This PR updates all references to data to data_.

[lglina]

FYI, this cropped up while trying to compile ArduinoJson.

[mike-matera]

Thank you so much!