Add ElectrumX get block headers method and tests

src/electrumx.js

@@ -391,6 +391,60 @@ class ElectrumX {
       else throw error
     }
   }
+
+  /**
+   * @api Electrumx.blockHeader()  blockHeader()
+   * @apiName ElectrumX Block headers
+   * @apiGroup ElectrumX
+   * @apiDescription Return block headers for a given height
+   *
+   * @apiExample Example usage:
+   * (async () => {
+   *   try {
+   *     let headers = await bchjs.Electrumx.blockHeaders(42);
+   *     console.log(headers);
+   *   } catch(error) {
+   *    console.error(error)
+   *   }
+   * })()
+   *
+   * headers = {
+   *  "success": true,
+   *  "headers": [
+   *    "010000008b52bbd72c2f49569059f559c1b1794de5192e4f7d6d2b03c7482bad0000000083e4f8a9d502ed0c419075c1abb5d56f878a2e9079e5612bfb76a2dc37d9c42741dd6849ffff001d2b909dd6",
+   *    "01000000f528fac1bcb685d0cd6c792320af0300a5ce15d687c7149548904e31000000004e8985a786d864f21e9cbb7cbdf4bc9265fe681b7a0893ac55a8e919ce035c2f85de6849ffff001d385ccb7c"
+   *  ]
+   * }
+   *
+   * (async () => {
+   *   try {
+   *     let headers = await bchjs.Electrumx.blockHeaders(42, 1);
+   *     console.log(headers);
+   *   } catch(error) {
+   *    console.error(error)
+   *   }
+   * })()
+   *
+   * headers = {
+   *  "success": true,
+   *  "headers": [
+   *    "010000008b52bbd72c2f49569059f559c1b1794de5192e4f7d6d2b03c7482bad0000000083e4f8a9d502ed0c419075c1abb5d56f878a2e9079e5612bfb76a2dc37d9c42741dd6849ffff001d2b909dd6"
+   *  ]
+   * }
+   *
+   */
+  async blockHeader(height, count = 1) {
+    try {
+      const response = await axios.get(
+        `${this.restURL}electrumx/block/headers/${height}?count=${count}`,
+        _this.axiosOptions
+      )
+      return response.data
+    } catch (error) {
+      if (error.response && error.response.data) throw error.response.data
+      else throw error
+    }
+  }
 }
 
 module.exports = ElectrumX