Avoid extraneous space between visibility kw and ident for statics

Today, given a static like `static mut FOO: usize = 1`, rustdoc would
emit `static mut  FOO: usize = 1`, as it emits both the mutability kw
with a space and reserves a space after the mutability kw. This patch
fixes that misformatting.

This patch also adds some tests for emit of other statics, as I could
not find an existing test devoted to statics.

src/test/rustdoc/static.rs

@@ -0,0 +1,12 @@
+// compile-flags: --document-private-items
+
+#![crate_type = "lib"]
+
+// @has static/static.FOO.html '//pre[@class="static"]' 'static FOO: usize'
+static FOO: usize = 1;
+
+// @has static/static.BAR.html '//pre[@class="static"]' 'pub static BAR: usize'
+pub static BAR: usize = 1;
+
+// @has static/static.BAZ.html '//pre[@class="static"]' 'pub static mut BAZ: usize'
+pub static mut BAZ: usize = 1;