go: time: Parse behaves inconsistently when parsing numerical timezones with an "MST" format string

What version of Go are you using (go version)?

Local machine:

$ go version
go version go1.12 darwin/amd64

Docker container:

$ go version
go version go1.12 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/nmooney/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/nmooney/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.12/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.12/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/vv/t3z4pbwn3pd36hm89fg1m8jc0000gn/T/go-build118701068=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Go Playground example here.

I attempted to use the time.Parse format string "Mon, 2 Jan 2006 15:04:05 MST" to parse the date "Tue, 12 Mar 2019 15:34:39 -0000".

What did you expect to see?

I would expect time.Parse to fail to parse the string, since it’s suffixed with -0000 rather than an alphabetical time zone designator.

This behaves correctly when I try to parse a date with a non-zero offset, such as "Tue, 12 Mar 2019 15:34:39 -0700" (i.e. parsing fails).

What did you see instead?

time.Parse successfully parses date strings with TZ offsets of zero given a format string that ends with MST, when it should likely fail if there is no alphabetical timezone designator.