fix(exports): fix calculating export is clause for the contacts

When contact has no extra space between the is and start of the contract body the bracket is counting
as the is part of the contact. For example:

`A is B{` the is clause will be `is B{` instead of `is B`

Closes #73

lib/exportsAnalyzer.ts

@@ -51,7 +51,7 @@ export interface ExportsAnalyzerResultUsingDirective {
 }
 
 export class ExportsAnalyzer {
-  constructor(private contents: string) {}
+  constructor(private contents: string) { }
 
   /**
    * Analyzes all the exports of the file (Contract, Interface, Library)
@@ -97,7 +97,7 @@ export class ExportsAnalyzer {
           name: e.name,
           body: this.contents.substring(e.body.start, e.body.end + 1).trim(),
           is: e.is
-            ? this.contents.substring(e.is.start, e.is.end + 1).trimLeft()
+            ? this.contents.substring(e.is.start, e.is.end).trimLeft() + ' '
             : '',
         });
       });

test/compiled/ContractWithoutSpaceBeforeBracket.sol

@@ -0,0 +1,5 @@
+contract B {
+}
+
+contract A is B {
+}

test/contracts/ContractWithoutSpaceBeforeBracket.sol

@@ -0,0 +1,4 @@
+contract B{
+}
+contract A is B{
+}

test/index.spec.ts

@@ -159,4 +159,8 @@ describe('Solidity Merger', () => {
       assert.isOk(e);
     }
   });
+
+  it('should compile file without extra spaces before bracket in contract defininition', async () => {
+    await testFile('ContractWithoutSpaceBeforeBracket');
+  });
 });