Add vectorised unit test for Pangram

As another demonstration of issue exercism#109, I've added a vectorised unit test to the Pangram exercise:
- Add vector test to unit tests.
- Rename `is_pangram` function as `is_pangram_scalar`.
- Create new `is_pangram` function by calling `Vectorize` on `is_pangram_scalar` function.

exercises/pangram/example.R

@@ -1,6 +1,6 @@
 library(magrittr)
 
-is_pangram <- function(input) {
+is_pangram_scalar <- function(input) {
 
   input_letters <- gsub("[^A-Za-z]", "", input) %>% 
                     tolower() %>% 
@@ -12,3 +12,5 @@ is_pangram <- function(input) {
   identical(input_letters, letters)
 
 }
+
+is_pangram <- Vectorize(is_pangram_scalar)

exercises/pangram/test_pangram.R

@@ -36,9 +36,26 @@ test_that("pangram with mixed case and punctuation", {
     expect_true(is_pangram("\"Five quacking Zephyrs jolt my wax bed.\""))
 })
 
-test_that("upper and lower case versions of the same character should not be 
+test_that("upper and lower case versions of the same character should not be
           counted separately", {
     expect_false(is_pangram("the quick brown fox jumped over the lazy FOX"))
 })
 
+test_that("The submission can handle vector inputs", {
+    expect_true (all (
+      is_pangram(c(
+        "",
+        "the quick brown fox jumps over the lazy dog",
+        "a quick movement of the enemy will jeopardize five gunboats",
+        "the quick brown fish jumps over the lazy dog",
+        "the_quick_brown_fox_jumps_over_the_lazy_dog",
+        "the 1 quick brown fox jumps over the 2 lazy dogs",
+        "7h3 qu1ck brown fox jumps ov3r 7h3 lazy dog",
+        "\"Five quacking Zephyrs jolt my wax bed.\"",
+        "the quick brown fox jumped over the lazy FOX")) ==
+        c(FALSE, TRUE, FALSE, FALSE, TRUE, TRUE, FALSE, TRUE, FALSE)
+      )
+    )
+})
+
 message("All tests passed for exercise: pangram")