Support for build args with space

[AndreaGiardini]

If a build argument contains the space character, that's replaced by the + character.

This is due to

dockerfile-maven/plugin/src/main/java/com/spotify/plugin/dockerfile/BuildMojo.java

Lines 291 to 297 in 986f960

	private static String encodeBuildParam(Object buildParam) throws MojoExecutionException {
	try {
	return URLEncoder.encode(new Gson().toJson(buildParam), "utf-8");
	} catch (UnsupportedEncodingException e) {
	throw new MojoExecutionException("Could not build image", e);
	}
	}

that encodes build arguments using URLEncoder. Indeed, we can read here that URLEncoder replaces the space character " " with a plus sign "+".

After the encoding, I added a replace to substitute the + sign with a space: after the encoding all the plus signs will be already encoded to /0x2B, so the ones left are the ones are coming from spaces.

I expanded the test case to make sure that everything will continue to work in the future.
Let me know if you need additional info.

Fixes #136
Fixes #286

[andrewegel]

Hi there - I too encountered this issue - It saddens me to (sighs as he realizes hes working with OSS) see this at the top of the project:

At this point, we're not developing or accepting new features or even fixing non-critical bugs.

Is there a separate issue for finding another maintainer? This deviated behavior from Docker is somewhat concerning.