fix(mathjax): remove blank line item and update test case

partial fix for issue2 of #8.

src/lib/turndown-plugin-mathjax.js

@@ -9,7 +9,7 @@ rules.removeDisplayFormula = {
 };
 
 // remove inline formula
-rules.removeInlineFormulat = {
+rules.removeInlineFormula = {
   filter: node => {
     return (
       node.nodeName === "SPAN" &&
@@ -20,6 +20,37 @@ rules.removeInlineFormulat = {
   replacement: () => ""
 };
 
+rules.removeBlankListItem = {
+  filter: ["li"],
+  replacement: function(content, node, options) {
+    node.childNodes.forEach(child => {
+      if (child.className && child.className.startsWith("MathJax")) {
+        node.removeChild(child);
+      }
+    });
+    //console.log(content);
+    content = content
+      .replace(/^\n+/, "") // remove leading newlines
+      .replace(/\n+$/, "\n") // replace trailing newlines with just a single one
+      .replace(/\n/gm, "\n    "); // indent
+    let prefix = options.bulletListMarker + "   ";
+    const parent = node.parentNode;
+    if (parent.nodeName === "OL") {
+      const start = parent.getAttribute("start");
+      const index = Array.prototype.indexOf.call(parent.children, node);
+      prefix = (start ? Number(start) + index : index + 1) + ".  ";
+    }
+    if (content !== "") {
+      return (
+        prefix +
+        content +
+        (node.nextSibling && !/\n$/.test(content) ? "\n" : "")
+      );
+    }
+    return "";
+  }
+};
+
 // remove new lines surrounding blank paragraph before script node
 rules.paragraphBeforeScriptNode = {
   filter: "p",

test/fixtures/issue-8-2.md

@@ -9,4 +9,4 @@ LDA出来说话了：**主题和词汇的分布就是多项式分布**！因为
 -   $\vec{\theta_m}$代表第$m$篇文章的多项式分布参数，长度为$K$，因此$\Theta$是一个-   $M*K$的矩阵，没一行代表一篇文章的多项式分布参数
 -   $N_m$代表第-   $m$篇文章的长度
 -   $z_{m,n}$代表第$m$篇文章第-   $n$个词由哪个主题产生的
--   $w_{m,n}$代表第$m$篇文章第$n$个词 copy.js:95:7
+-   $w_{m,n}$代表第$m$篇文章第$n$个词