(This is just a copy of #5169 that was closed automatically by mistake due to referencing from another PR)

This PR has potential breaking changes in AxiosHeaders class.

• this PR fixes a bug with defaults merging (Content type changed from multipart/form-data to application/x-www-form-urlencoded #5067, User-Agent set in global and instance defaults are not respected #5157) and Restricted in strict mode #5006
• AxiosHeaders signature has been refactored to an ES6 class
• constructor signature has been changed - removed the second argument (defaultHeaders);
• added a handy concat method to merge headers into a new instance. This is a functional replacement to merge with defaults externally. This change greatly simplifies the implementation and makes class usage more transparent.

const headersA = new AxiosHeaders({x:1});
const headersB = headersA.concat({y:2}, {z:3}, 'foo: 1\nbar: 2');
const headersC = AxiosHeaders.concat({x:1}, {y:2}, {z:3})

• set method (as well as the class constructor) now accepts a AxiosHeaders instance and raw headers string as the source
• added instance iterator to iterate over resolved headers
• added toString to serialize into a raw headers string
• default content type now is undefined, not application/x-www-form-urlencoded. This step makes it easier to keep track of user-defined header values, simplifying the implementation of the AxiosHeaders class. However, the content header will still be added before the request is sent if it is not explicitly defined or disabled by the user. Only the place and method of adding this header inside the Axios have been changed.
• AxiosHeaders instance can be used as a header option value;
• config header merging is now caseless;

Closes #5157;
Closes #5067;
Closes #5006;
Closes #5226;
Closes #4998;