Fix #946 (#949)

* Fix #946

Issue #946 identified a case in which `array.js` ate the final row of a well-written `aligned` environment.

This PR modifies code from PR #479 to fix this problem and to also continue to eat a trailing `\\` submitted to any environment.

* Fix bug and add tests

* Add final newline to test.

Doh!

test/katex-spec.js

@@ -1251,6 +1251,11 @@ describe("A begin/end parser", function() {
     it("should allow \\cr as a line terminator", function() {
         expect("\\begin{matrix}a&b\\cr c&d\\end{matrix}").toParse();
     });
+
+    it("should eat a final newline", function() {
+        const m3 = getParsed("\\begin{matrix}a&b\\\\ c&d \\\\ \\end{matrix}")[0];
+        expect(m3.value.body.length).toBe(2);
+    });
 });
 
 describe("A sqrt parser", function() {
@@ -2409,6 +2414,10 @@ describe("An aligned environment", function() {
             .toNotParse();
     });
 
+    it("should not eat the last row when its first cell is empty", function() {
+        const ae = getParsed("\\begin{aligned}&E_1 & (1)\\\\&E_2 & (2)\\\\E_3 & (3)\\end{aligned}")[0];
+        expect(ae.value.body.length).toBe(3);
+    });
 });
 
 describe("A parser that does not throw on unsupported commands", function() {