textDocument/documentHighlight doesn't highlight variables if they're redeclared

[rcjsuen]

FROM node
ARG var=value
ARG var=value2
RUN echo $var
RUN echo $var

If you select the $var on the two RUN lines, only the second ARG instruction gets highlighted. The first one doesn't.

[rcjsuen]

Reopening because I wrote the wrong commit message in 98601e9. It should have closed #141, not this one.