Properly use exepath for CheckBinPath (#864)

* Properly use exepath for CheckBinPath

 #823 added this the wrong way, it was reverted back on 9f0cf00 but still not achieving the right functionality. This uses it properly now, also adding it to how we lookup goimports for srcdir support.

autoload/go/fmt.vim

@@ -87,7 +87,7 @@ function! go#fmt#Format(withGoimport)
 
     " get the command first so we can test it
     let fmt_command = g:go_fmt_command
-    if a:withGoimport  == 1 
+    if a:withGoimport  == 1
         let fmt_command  = g:go_goimports_bin
     endif
 
@@ -109,13 +109,13 @@ function! go#fmt#Format(withGoimport)
 
     " populate the final command with user based fmt options
     let command = fmt_command . ' -w '
-    if a:withGoimport  != 1 
+    if a:withGoimport  != 1
         let command  = command . g:go_fmt_options
     endif
 
     if fmt_command == "goimports"
         if !exists('b:goimports_vendor_compatible')
-            let out = go#util#System("goimports --help")
+            let out = go#util#System(bin_path . " --help")
             if out !~ "-srcdir"
                 call go#util#EchoWarning("vim-go: goimports does not support srcdir. update with: :GoUpdateBinaries")
             else

autoload/go/path.vim

@@ -151,6 +151,9 @@ function! go#path#CheckBinPath(binpath)
 
     " if it's in PATH just return it
     if executable(binpath)
+        if v:version == 704 && has('patch235')
+            let binpath = exepath(binpath)
+        endif
         let $PATH = old_path
         return binpath
     endif