add explicit std::array<> template args to support C++14 build

Not all the build systems make it easy to specify the C++ standard
to use (somehow, Bazel doesn't). C++14 doesn't support deducing
the template arguments for std::array. But it seems like there's
only one place that needs to be changed to make a C++14 build
work ok.

Author: johnbartholomew

core/unicode_test.cpp

@@ -59,7 +59,7 @@ TEST(Unicode, TestUTF8)
 
 TEST(Unicode, TestUTF8RejectBad)
 {
-    const auto test_cases = std::array{
+    const auto test_cases = std::array<const char*, 20>{
         "\x80",  // Continuation byte without leading byte
         "\xa0",  // Continuation byte without leading byte
         "\xbf",  // Continuation byte without leading byte