Regression in 0.0.7 causes ARG's getProperty() to return different things

[rcjsuen]

The change for PropertyInstructions to not override getArguments() in c7416ab has caused a regression. The following code shouldn't make arg.getProperty() return null.

var dockerfileAst = require("dockerfile-ast");
var dockerfile = dockerfileAst.DockerfileParser.parse("ARG a=\"a\\  b");
var arg = dockerfile.getInstructions()[0];
console.log(arg.getProperty() !== null);

Building against dockerfile-ast@0.0.8:

$ docker build .
Sending build context to Docker daemon   1.78MB
Step 1/4 : FROM node:alpine
 ---> 0a3c080a624d
Step 2/4 : RUN npm install dockerfile-ast@0.0.8
 ---> Using cache
 ---> 9f7a63b9815d
Step 3/4 : COPY index.js index.js
 ---> 40863e9828a3
Step 4/4 : RUN node index.js
 ---> Running in 56e7bcad6106
false
Removing intermediate container 56e7bcad6106
 ---> 6be49449a6bb
Successfully built 6be49449a6bb

Building against dockerfile-ast@0.0.7:

$ docker build .
Sending build context to Docker daemon   1.78MB
Step 1/4 : FROM node:alpine
 ---> 0a3c080a624d
Step 2/4 : RUN npm install dockerfile-ast@0.0.7
 ---> Using cache
 ---> b1e26d38c852
Step 3/4 : COPY index.js index.js
 ---> 07f29466745a
Step 4/4 : RUN node index.js
 ---> Running in bcaf8b1186e6
false
Removing intermediate container bcaf8b1186e6
 ---> fac0b5cfaa6d
Successfully built fac0b5cfaa6d

Building against dockerfile-ast@0.0.6:

$ docker build .
Sending build context to Docker daemon   1.78MB
Step 1/4 : FROM node:alpine
 ---> 0a3c080a624d
Step 2/4 : RUN npm install dockerfile-ast@0.0.6
 ---> Using cache
 ---> d53738fe0e50
Step 3/4 : COPY index.js index.js
 ---> e0ad583854a4
Step 4/4 : RUN node index.js
 ---> Running in e1e1de560b31
true
Removing intermediate container e1e1de560b31
 ---> 87501b7cf644
Successfully built 87501b7cf644