Check type of callCount argument and error accordingly

This is to fixes sinonjs#2408, which could result in error messages like
"expected spy to be called 10 times but was called 10 times".

Now we will instead say "expected '10' to be a number, but was of type
string", which is much clearer!

lib/sinon/assert.js

@@ -159,10 +159,16 @@ function createAssertObject() {
         callCount: function assertCallCount(method, count) {
             verifyIsStub(method);
 
-            if (method.callCount !== count) {
-                var msg = `expected %n to be called ${timesInWords(
-                    count
-                )} but was called %c%C`;
+            var msg;
+            if (typeof count !== "number") {
+                msg =
+                    `expected ${format(count)} to be a number ` +
+                    `but was of type ${typeof count}`;
+                failAssertion(this, msg);
+            } else if (method.callCount !== count) {
+                msg =
+                    `expected %n to be called ${timesInWords(count)} ` +
+                    `but was called %c%C`;
                 failAssertion(this, method.printf(msg));
             } else {
                 assert.pass("callCount");

test/assert-test.js

@@ -1570,6 +1570,15 @@ describe("assert", function () {
             );
         });
 
+        it("assert.callCount exception message with non-numeric argument", function () {
+            this.obj.doSomething();
+
+            assert.equals(
+                this.message("callCount", this.obj.doSomething, "3"),
+                "expected '3' to be a number but was of type string"
+            );
+        });
+
         it("assert.calledOnce exception message", function () {
             this.obj.doSomething();
             this.obj.doSomething();