False positive flagged for quotes in ENV and LABEL instructions

[rcjsuen]

If a LABEL instruction uses quotes for its values, we seem to be flagging it as an error even though it builds just fine. Perhaps this was caused by the fix for #40?

FROM scratch
LABEL "a"="b" \
    "c"="d" \
    "e"="f"

$ docker build .
Sending build context to Docker daemon  1.774MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : LABEL "a"="b"     "c"="d"     "e"="f"
 ---> Running in 9b71e47cfa13
Removing intermediate container 9b71e47cfa13
 ---> 7b64e135ec7f
Successfully built 7b64e135ec7f

The line numbers in the error also seem weird...

$ dockerfile-utils lint Dockerfile
Line: 4-3
Error: Syntax error - can't find = in "    ". Must be of the form: name=value

Line: 4
    "c"="d" \
       ^^^^
Error: LABEL names can not be blank

Line: 5-4
Error: Syntax error - can't find = in "    ". Must be of the form: name=value

Line: 5
    "e"="f"
       ^^^^
Error: LABEL names can not be blank

[rcjsuen]

This is probably related and/or caused by rcjsuen/dockerfile-ast#45.