patch 8.2.0427: it is not possible to check for a typo in a feature name

Problem:    It is not possible to check for a typo in a feature name.
Solution:   Add an extra argument to has().

src/testdir/check.vim

@@ -6,6 +6,9 @@ command -nargs=1 MissingFeature throw 'Skipped: ' .. <args> .. ' feature missing
 " Command to check for the presence of a feature.
 command -nargs=1 CheckFeature call CheckFeature(<f-args>)
 func CheckFeature(name)
+  if !has(a:name, 1)
+    throw 'Checking for non-existent feature ' .. a:name
+  endif
   if !has(a:name)
     MissingFeature a:name
   endif

src/testdir/test_functions.vim

@@ -20,6 +20,14 @@ func Test_00_bufexists()
   call assert_equal(0, bufexists('Xfoo'))
 endfunc
 
+func Test_has()
+  call assert_equal(1, has('eval'))
+  call assert_equal(1, has('eval', 1))
+
+  call assert_equal(0, has('nonexistent'))
+  call assert_equal(0, has('nonexistent', 1))
+endfunc
+
 func Test_empty()
   call assert_equal(1, empty(''))
   call assert_equal(0, empty('a'))
@@ -1586,7 +1594,7 @@ func Test_confirm()
   call assert_equal(2, a)
 
   " confirm() should return 0 when pressing CTRL-C.
-  call feedkeys("\<C-c>", 'L')
+  call feedkeys("\<C-C>", 'L')
   let a = confirm('Are you sure?', "&Yes\n&No")
   call assert_equal(0, a)
 

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    427,
 /**/
     426,
 /**/